Most lighting level measurements are characterized in terms of illuminance. While this is useful for indoor applications, illuminance levels are not always as useful for outdoor lighting efficiency characterization. Lighting designers and researchers need a method to accurately characterize their outdoor or low level lighting applications.  This invention is directed to a system for approximating the flux density of light on a human retina. Different tasks require different structures of the eye. For example, visual acuity tasks require the use of the foveal cones, while movement in the peripheral vision is detected by rods in the retina.  This system includes a housing unit that replicates the structure of the eye, including two detectors to detect light inside the housing. One of these detectors simulates cones to produce a photopic spectral response, while the other detector replicates the scotopic spectral response of rods.  A processor in the system calculates a mesopic flux density of the light inside the housing based on these responses.